Lexical scoping is not what I expect

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Prof Brian Ripley
Sent: Friday, June 28, 2013 11:16 AM
To: r-help at r-project.org
Subject: Re: [R] Lexical scoping is not what I expect

On 28/06/2013 15:54, John Fox wrote:

Dear Duncan and Steve,

Since Steve's example raises it, I've never understood why it's legal

to

change the built-in global "constants" in R, including T and F. That

just

seems to me to set a trap for users. Why not treat these as reserved
symbols, like TRUE, Inf, etc.?

Because people wanted to use them as names of things.  Maybe a T (not
t)
statistic.

And BTW, you can change the value of T for yourself, but you will not
change it for any package code, including R itself, since the base
namespace is ahead of the workspace in namespace scoping.  Because of
that, it is years since I have seen anyone actually trip themselves up
over this.

Best,
  John

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Duncan Murdoch
Sent: Friday, June 28, 2013 9:40 AM
To: S Ellison
Cc: r-help at r-project.org
Subject: Re: [R] Lexical scoping is not what I expect

On 28/06/2013 9:28 AM, S Ellison wrote:

I too find R's lexical scoping rules straightforward.
However, I'd say that if your code relies on lexical
scoping to find something, you should probably rewrite your code.

Except of course that almost every function relies on lexical
scoping to some extent!

This could get messy, because a) that's true and b) it actually

leads

to some genuine risks when 'globals' get redefined or masked*.

How about I amend the assertion to "if your code relies on lexical

scoping to find a variable you defined, you should probably rewrite
your code."

and leave it at that, subject to some common sense about whether

you

know what you're doing?

That still isn't right, because users should feel free to define
functions and call them from their other functions.

I think who defined it isn't the issue, the issue is whether it

might

change unexpectedly.  The user owns globalenv().  The package author
owns the package namespace.  So packages should almost never read or
write things directly from/to globalenv() (the user might change

them),

but they can create their own private environments and write there.

Where it gets a little less clear is when the user writes a

function.

I
would say functions should never write directly to globalenv(), but
it's
perfectly fine to reference constants there (like other functions
written by the user).  Referencing things there that change is the
risky
thing.

Duncan Murdoch

Steve E


*Example

sin.deg  <- function(deg) sin(deg * pi/180)
sin.deg(45)

[1] 0.7071068
	#looks about right

pi <- 3.2       #Indiana General Assembly bill #247, 1897.
sin.deg(45)

[1] 0.7173561
	#oops ...




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--
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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